Grid bias arrangements in thermionic valve circuits



y 1934. 1.. 1.. DE KRA'M'OLIN 1,967,917

GRID BIAS ARRANGEMENTS .[N THERMIONIC VALVE CIRCUITS Filed July 19, 1929 Patented July 24, 1934 TES GRID BIAS ARRANGEMENTS IN THERMI- ONIC VALVE CIRCUITS Lon Ladislas de Kramolin, Berlin-Pankow, Germany Applica In Germany 7 Claims.

This invention relates to multi-tube amplihere, some or all of the tubes of which have equipotential cathodes.

One object of the invention is the provision of improved means in such apparatus for obtaining the desired grid bias without giving rise to backcoupling between the various stages of the amplifier.

Other objects and advantages will appear hereinafter.

It is the general practice, in mains driven radio receiving sets, to take the necessary grid bias potential for each of the tubes of a multi-tube amplifier from a common resistance situated in the lead from the negative terminal of the source of high tension. Experiments have shown, however, that this common resistance acts, particularly in the case of amplifiers of which all the stages work at the same frequency, such as multistage high frequency or multi-stage low frequency amplifiers, as a coupling between the various grid circuits. As is well-known it has been attempted to overcome this difliculty by the use of large condensers (of several micro-farads capacity) in parallel with the resistance. Apart from their high cost, however, condensers of this type are undesirable.

The present invention provides a means of avoiding this coupling effect in a multi-tube amplifier, without necessitating the use of condensers for this purpose, so as to provide an amplifier that is free not only from alternating current hum but also from back-coupling.

The invention consists essentially in the substitution for the single common resistance usually employed, of a separate resistance for each tube of the amplifier.

The heating current is preferably taken from a common heating winding for all of the tubes and the individual resistance for each tube connected between the respective cathode and a point connected both to the mid-point of the heating winding and the high tension negative terminal.

Two embodiments of the invention are shown in the accompanying drawing in which:

Figure 1 shows the invention applied to three indirectly heated cathode tubes supplied from the mains,

Figure 2 shows the invention applied to an indirectly and a directly heated cathode tube used in combination.

Referring to Figure 1: The three tubes 1 have their filaments 3 supplied from the mains through a transformer 4. The rectangles 5 in the grid tion July 19, 1929, Serial No. 379,468

July 27, 1928 and anode circuits represent any desired intervalve coupling. A lead 6 from the negative terminal of the source of high tension is connected to the centre point of the secondary winding of the transformer 4 an resistances '7 the other ends of which are connected to the cathodes 3 of the respective tubes. Owing to the difference of potential between the filaments 3 and the cathodes 8, there is a fall of potential along each of the resistances 7, which thus serve as potcntiometers from which the necessary biasing potential for the grids 9 may be taken.

In the construction shown in Figure 2, the grid biasing potential for the directly heated cathode valve 2 is taken from a resistance 11 included in the lead 6 from the negative terminal of the source of high tension. The grid biasing potential for the indirectly heated cathode tube 1 is obtained from the resistance 7 as described with reference to Figure 1.

What I claim and desire to Patent is:-

1. In a mains-energized radio receiving apparatus employing at least one indirectly heated tube and one directly heated tube, the combination of a common heating winding for said tubes, an individual resistance for each tube connected in the negative part of the cathodeplate circuit of the respective tube, and means connecting the grids of said tubes spective resistances for obtaining the necessary grid bias.

2. In a mains-energized apparatus employing an indirectly heated thermionic tube and a directly heated thermionic tube, the combination of a heating winding common to both indirectly and directly heated tubes, an individual resistance for each tube connected in the negative secure by Letters part of the respective cathode-plate circuit and electrically connected to the mid-point of said eating winding and means connecting the grids to the respective resistances for obtaining the necessary grid bias.

3. In a mains-energized apparatus employd to the three parallel to the reing an indirectly heated thermionic tube and a directly heated thermionic tube, the combination of a heating winding common to both indirectly and directly heated tubes, an individual resistance for the indirectly heated tube connected between the cathode and a point connected to both said heating winding and the high tension negative, an individual resistance for the directly heated tube connected between said heating winding and high tension negative, and

means connecting the "grids to the respective resistances for obtaining the necessary grl'd bias.

4. In a multi-stage amplifier, a plurality er thermionic tubes connected in cascade for amplifying the same incoming signal frequency, at least one of said tubes being provided with an equi-potenial cathode, a common heating aircuit for the cathodes of said tubes, a" source of plate current common to said tubes, inter-stage coupling devices connected in the plate leads from said source, means'for-bias'ing the grids of said tubes comprising resistances each of Which is entirely individual to a single tube and is connected between the cathode of its corresponding tube and the negative terminal of the plate current source, and means connecting the grids of the tubes to the respective resistances for obtaining the desired grid bias.

5. In a mains energized multi-tube amplifier, 'a'pluralitv of thermionic tubes, having, equipotential cathodes, said tubes being connected in cascadefor operation at approximately'theisame signal frequency, a mains transformer having a heater winding common to all the heaters of said cathodes, a source of plate-current common to said tubes, inter-stage coupling devices connected in the plate leads "from said source, a

current, andmeans for connectingthe grids of the tubes to the respective resistances for obtaining the desired grid bias, said resistances comprising the sole means for obtaining the desired bias on the grids of the tubes,

if? it" sired bias on ithegrids of the tubes."

6. In a mains energized multi-tube amplifier, a plurality of thermionic tubes having equipo tential cathodes all of the tubes being 0011- nected in cascade for operation at approximately the same signal frequency, a source of plate current common to said tubes, inter stage couplingdevices connected mime plate leads from said source, a mains transformer havinga heat er'winding common to all the heaters of said cathodes, a resistance individual to each tube and connected between the cathode of its cor responding tube anda point connected toboth the mid-point of said heater winding and the negativeterminal of said plate current source, and means for connecting the grids of the tubes to the respective resistances for obtaining the desired Lgrid .bias, said resistances comprising the sole means for obtaining the desired bias on the, grids of the tubes.

'7. In a mains energized multi-tube amplifier, a plurality of thermionic tubeshaving equipotential'cathodes, said tubes being-connected in cascade for operation at approximately the same a source of heatercurrent commonto"all the heaters of said cathodes, a source of plate cur- 109 grids of the tubesito the respectiveresistancesffor obtaining the desired grid bias, said resistances, comprising the sole means for obtaining the de- :LE'ONV p s s DE ML Ni. 1:10 

